Critical fastener locations on aircraft platforms require regular inspection to verify that cracking, corrosion or other deleterious conditions are not present. Also, critical locations on other structures, such as bridges, piping systems, pipelines, railroad rails, at mounting locations for vessels, drive trains, pumps and other structures and facilities require either inspections for damage or operational variable monitoring. Operational variables of interest include temperature and stresses/loads at critical interfaces and at varied depths into materials near joints, mounts and at other critical locations.
Inspection can require disassembly to gain access to critical locations. This disassembly often requires extensive man-hours incurring costs and operational down time and often causing collateral damage. Because of these costs and the impact on readiness and availability, inspections are performed as infrequently as possible while maintaining an acceptable level of risk. Thus, by the time damage is detected, it is often after considerable damage has occurred and parts must be replaced rather than repaired. Also, most inspections require removal of protective coatings, insulation and neighboring structures. Since many of these coatings, insulation layers and critical structures where deposited or installed by the original equipment manufacturers or during large-scale production or maintenance events, the replacement of these coatings, insulation and structures after inspections is often performed at a substantially poorer level of quality than the original deposition or installation. Thus, it is the previous inspection locations that often become the most likely damage sites. For example, a component with a corrosion protection coating or a sealed enclosure that has been removed or unsealed but not replaced at the same quality level may allow the corrosive environment to interact with susceptible materials resulting in severe corrosion that impacts safety critical structures and is revealed during the next inspection. This may require the part to be retired and may represent a significant safety hazard.
If a corroded part does not exceed the establish damage limit, corrosion may be blended out and the part returned to service. Bolt holes with small cracks detected through inspection may be repaired by oversizing the hole and installing bushings. Regions away from holes may be repaired by simply blending the corrosion and then treating, priming, and painting the exposed metal or by using advanced methods such as cold spray to replace the lost metal and reinstate the material to its original structural integrity. Flat surfaces may be repaired by machining the surface flat and installing a shim to return the surface to its original shape, using an epoxy to fill blended and dimpled surfaces, or simply leaving the blended out divots and scooped out material as machined. These repairs are susceptible to water entrapment or intrusion that may lead to corrosion if not repaired properly. In many aerospace applications these corrosion-prone areas are where flight controls and gearbox mount feet attach, and the load path through these locations is critical for aircraft safety. This is also true in other structures and facilities common in refineries, rail cars and railroad rails, bridges, power plants, piping systems, and ship systems.